Analog to digital conversion circuit

ABSTRACT

An analog to digital (AD) converter includes an AD conversion circuit, and a calibration circuit that calibrates an output value of the AD conversion circuit. The calibration circuit includes a right-shift circuit that shifts an accumulated value of values obtained by removing a deviated value from a plurality of output values of the AD conversion circuit. The calibration circuit calibrates the output value of the AD conversion circuit based on the shifted value.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of U.S. patentapplication Ser. No. 15/799,034, filed on Oct. 31, 2017, which is aContinuation Application of U.S. patent application Ser. No. 15/615,719,filed on Jun. 6, 2017, now U.S. Pat. No. 9,838,027 B2, issued on Dec. 5,2017, which is a Continuation Application of U.S. patent applicationSer. No. 15/371,101, filed on Dec. 6, 2016, now U.S. Pat. No. 9,698,804,issued on Jul. 4, 2017 which claims priority from Japanese PatentApplication No. 2016-000824 filed on Jan. 6, 2016 including thespecification, drawings and abstract incorporated herein by reference inits entirety.

BACKGROUND

The present invention relates to an analog to digital convertor, and inparticular, can be preferably used for an analog to digital convertoraccompanied by calibration.

In an analog to digital convertor (ADC), linearity caused by mismatch ofelements in the ADC and a conversion error due to offset have beengenerally corrected using a digital calibration technique. Here, the“analog to digital convertor” is referred to an analog to digitalconversion circuit, an AD convertor, an AD conversion circuit, or an ADCin some cases. Further, “AD” is written as “A/D” in some cases. Further,the “mismatch of elements” includes mismatch with respect to a designvalue, namely, a relative error and relative variation generated whenthe elements are designed to be supposed to have the same elementalvalue or elemental values with a specified ratio, in addition tomanufacturing variation and an absolute error. In order to remove anerror of a correction value generated due to noise such as elementalnoise in calibration, correction values are obtained by a plurality ofoperations, and the final correction value is obtained by averaging theoperation results of the correction values. Alternatively, the finalcorrection value is similarly obtained in general by an operation bywhich the same effect can be obtained, for example, by allowing theoperation results to pass through a low-pass filter or by accumulatingthe operation results.

Japanese Unexamined Patent Application Publication No. 2004-222274discloses a circuit technique of correcting an error of an outputvoltage of a digital to analog convertor and a gain error of anamplifying circuit in a pipeline stage of a pipeline-type ADC. FIG. 2 ofthe literature shows an error correction data generation circuit, a DACerror correction circuit, and a gain error correction circuit, and aneffect of noise can be reduced by arranging an averaging circuit on theinput side or the output side of the error correction data generationcircuit. It should be noted that the “digital to analog convertor” isreferred to as a digital to analog conversion circuit, a DA convertor, aDA conversion circuit, or a DAC in some cases. Further, “DA” is writtenas “D/A” in some cases.

“S. Y. Chuang and T. L. Sculley, “A digitally self-calibrating 14-bit10-MHz CMOS pipelined A to D converter”, Journal of Solid-State Circuit,vol. 37, No. 6, June 2002” discloses a pipeline-type ADC in which anerror correction circuit is provided in each stage (see FIG. 1 of theliterature), and a calibration circuit coupled to the output of the ADCshown in FIG. 6 of the literature includes an averaging circuit (2048averaging and ALU circuit).

A calibration method in which an element parameter in an analog circuitis finely adjusted by feeding back an operation result in a digitalregion to an analog region has been generally used. For example, thereis a self-trimming circuit shown in FIG. 3 of “S. T. Ryu, “A 14b-LinearCapacitor Self-Trimming Pipelined ADC”, Journal of Solid-State Circuit,vol. 39, No. 11, November 2004”.

SUMMARY

As a result of examining Japanese Unexamined Patent ApplicationPublication No. 2004-222274, “S. Y. Chuang and T. L. Sculley, “Adigitally self-calibrating 14-bit 10-MHz CMOS pipelined A to Dconverter”, Journal of Solid-State Circuit, vol. 37, No. 6, June 2002”,and “S. T. Ryu, “A 14b-Linear Capacitor Self-Trimming Pipelined ADC”,Journal of Solid-State Circuit, vol. 39, No. 11, November 2004”, theinventors found the following new problems.

FIG. 1 is a general and conceptual block diagram of an analog to digitalconversion circuit (signal processing circuit) having a function ofcalibrating an error generated due to variation or the like in an analogregion using an operation process in a digital region. A calibrationcircuit 3 is coupled to the digital output of an ADC 1 into which ananalog input VIN is input. The ADC 1 and the calibration circuit 3 arearranged in the analog region and the digital region, respectively, andentirely configure a signal processing circuit 9. The entire signalprocessing circuit 9 may be referred to as an AD convertor or ADconversion circuit having a calibration function.

The calibration circuit 3 includes a correction circuit 7, an averagingcircuit 8, and a correction value operation circuit 6. In a calibrationoperation, a correction value is obtained from the digital output of theADC 1 by the correction value operation circuit 6 a plurality of timesto be supplied to the averaging circuit 8. The averaging circuit 8obtains an average value, as the final correction value, from thecorrection values. The final correction value is held, and is used forcorrecting the digital output of the ADC 1 with the correction circuit 7in a normal operation that follows.

An effect of random noise such as elemental noise in the analog regioncan be removed by using a value obtained by averaging the operationresults of the correction value operation circuit fixed times or more asa correction value. However, in the case where the operation results ofthe correction value operation circuit 6 include a largely-differentresult due to generation of unexpected large external noise, the finalcorrection value is deviated. FIG. 2 are explanatory diagrams eachschematically showing an impact on the correction value when unexpectedlarge external noise is generated. FIG. 2A shows a case in which noiseis random noise and correction value operation results are normallydistributed, and FIG. 2B shows a case in which a correction valueoperation result with a large error is included due to unexpected largeexternal noise. In the case where noise in the ADC of the analog regionis random noise such as elemental noise, the correction value operationresults are expected to be normally distributed as shown in FIG. 2A, anda correct correction value is calculated by removing the noise byaveraging. In the case where the correction value operation resultshaving the normal distribution include a large value (the same appliesto a small value) that is deviated from a value that is supposed to be acorrection value due to unexpected external noise as shown in FIG. 2B,the correction value calculated by averaging includes an error deviatedfrom a value that is supposed to be a correction value.

In order to obtain a correct correction value using the averaged resulteven when large external noise is generated, it is necessary to averagean enormous number of correction value operation results, and a longperiod of time is required for calibration. Further, even in the casewhere unexpected external noise is frequently or periodically generatedduring the averaging in a long period of time, the effect cannot beremoved. In particular, in foreground calibration that determines acorrection value when starting the ADC 1, there is no opportunity tocorrect a wrong correction value, and a normal conversion result iscontinuously corrected using the wrong correction value. Thus, a correctconversion result cannot be obtained due to deterioration of linearityor superposition of offset on the conversion result.

It is conceivable that the calibration is performed in a noisycircumstance where external noise is largely generated in actual use,and the correction value is possibly deviated due to the unexpectedexternal noise as described above. However, a method of excluding theeffect of such unexpected noise is neither disclosed nor suggested inJapanese Unexamined Patent Application Publication No. 2004-222274, “S.Y. Chuang and T. L. Sculley, “A digitally self-calibrating 14-bit 10-MHzCMOS pipelined A to D converter”, Journal of Solid-State Circuit, vol.37, No. 6, June 2002”, and “S. T. Ryu, “A 14b-Linear CapacitorSelf-Trimming Pipelined ADC”, Journal of Solid-State Circuit, vol. 39,No. 11, November 2004”.

Means for solving such problems will be described below. However, theother problems and novel features will become apparent from thedescription of the specification and the accompanying drawings.

According to one embodiment, the disclosure is as follows.

Namely, provided is an analog to digital convertor including: an ADCthat converts an analog value of an input signal into a digital valueand outputs a converted value; and an averaging circuit that calculatesa correction value by a calibration operation. The converted value iscorrected and output using the correction value calculated and held bythe calibration operation in a normal operation. The analog to digitalconvertor is configured as follows.

In the calibration operation of the analog to digital convertor, anelemental correction value on the basis of a converted value by the ADCcorresponding to a predetermined analog value is supplied to theaveraging circuit a plurality of times. The averaging circuit calculatesthe average value of the remaining elemental correction values obtainedby removing, at least, the maximum value and the minimum value from theelemental correction values supplied a plurality of times, andcalculates the correction value on the basis of the average value.

The following is a description of a summary of an effect obtained in theembodiment.

Namely, even in the case where converted values of an ADC include alarge error due to unexpected external noise or the like during a periodof a calibration operation, a correction value can be calculated byexcluding the effect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general and conceptual block diagram of an analog to digitalconversion circuit accompanied by calibration;

FIG. 2 are explanatory diagrams each schematically showing an impact ona correction value of the calibration when unexpected large externalnoise is generated;

FIG. 3 is a block diagram for showing a configuration example of asignal processing circuit (AD convertor with a calibration circuit)according to a first embodiment;

FIG. 4 is a configuration diagram for showing an example of asemiconductor device on which the signal processing circuit (ADconvertor with the calibration circuit) is mounted;

FIG. 5 is a configuration diagram for showing another example of thesemiconductor device on which the signal processing circuit (ADconvertor with the calibration circuit) is mounted;

FIG. 6 is a block diagram for showing a configuration example of an ADconversion circuit of a sequential comparison system;

FIG. 7 is a circuit diagram for showing a configuration example of acapacitance DAC circuit of the sequential comparison-type AD conversioncircuit;

FIG. 8 is a block diagram for showing a configuration example of acorrection circuit;

FIG. 9 is a block diagram for showing a configuration example of acorrection value operation circuit;

FIG. 10 is a block diagram for showing a configuration example of anaveraging circuit having a maximum value/minimum value removal function;

FIG. 11 is a block diagram for showing a configuration example of amaximum value/minimum value removal circuit;

FIG. 12 is a block diagram for showing another configuration example ofthe maximum value/minimum value removal circuit;

FIG. 13 is an explanatory diagram for showing an operation example ofcalibration;

FIG. 14 is a circuit diagram for showing the states of switches in asampling state of the most significant bit in the capacitance DACcircuit in a calibration operation;

FIG. 15 is a circuit diagram for showing the states of the switches inthe sampling state of capacitors lower than the most significant bit inthe capacitance DAC circuit in the calibration operation;

FIG. 16 is a timing chart for showing an operation example in which thecorrection value of the capacitor corresponding to the most significantbit in the capacitance DAC circuit is calculated in the calibrationoperation;

FIG. 17 is a circuit diagram for showing the states of the switches inthe sampling state of the capacitor corresponding to the third bit fromthe most significant bit in the capacitance DAC circuit in thecalibration operation;

FIG. 18 is a circuit diagram for showing the states of the switches inthe sampling state of the capacitors lower than the third bit from themost significant bit in the capacitance DAC circuit in the calibrationoperation;

FIG. 19 is a timing chart for showing an operation example in which thecorrection values of the capacitors from the most significant bit to thethird bit in the capacitance DAC circuit are calculated in thecalibration operation;

FIG. 20 is a block diagram for showing a configuration example of thecorrection circuit adapted for the calibration in which the correctionvalues of the capacitors from the most significant bit to the third bitin the capacitance DAC circuit are calculated;

FIG. 21 is a block diagram for showing a configuration example of theaveraging circuit having a maximum value/minimum value removal functionadapted for the calibration in which the correction values of thecapacitors from the most significant bit to the third bit in thecapacitance DAC circuit are calculated;

FIG. 22 is a block diagram for showing a configuration example of anaveraging circuit having a maximum value/minimum value removal functionaccording to a second embodiment;

FIG. 23 is a block diagram for showing a configuration example of anaveraging circuit having a maximum value/minimum value removal functionaccording to a third embodiment;

FIG. 24 is a block diagram for showing a first configuration example ofan averaging circuit having a maximum value/minimum value removalfunction according to a fourth embodiment;

FIG. 25 is a block diagram for showing a second configuration example ofthe averaging circuit having a maximum value/minimum value removalfunction according to the fourth embodiment;

FIG. 26 is a block diagram for showing a third configuration example ofthe averaging circuit having a maximum value/minimum value removalfunction according to the fourth embodiment;

FIG. 27 is a block diagram for showing a configuration example of an ADconvertor with an offset calibration circuit that is a signal processingcircuit according to a fifth embodiment;

FIG. 28 is a block diagram for showing a configuration example of acorrection circuit according to the fifth embodiment;

FIG. 29 is a block diagram for showing a configuration example of anaveraging circuit having a maximum value/minimum value removal functionaccording to the fifth embodiment;

FIG. 30 is a circuit diagram for showing the states of switches in thecapacitance DAC circuit in a sampling state in an offset calibrationoperation;

FIG. 31 is a timing chart for showing an operation example of the offsetcalibration;

FIG. 32 is a block diagram for showing a configuration example of an ADconvertor to which both of a capacitance mismatch calibration circuitand an offset calibration circuit are coupled in a signal processingcircuit according to a sixth embodiment;

FIG. 33 is a timing chart for showing an operation example ofcalibration of the signal processing circuit according to the sixthembodiment;

FIG. 34 is an explanatory diagram for showing an operation example ofthe calibration of the sixth embodiment;

FIG. 35 is a block diagram for showing a configuration example of an ADconversion circuit of a pipeline system;

FIG. 36 is a block diagram for showing a configuration example of eachstage of the AD conversion circuit of a pipeline system; and

FIG. 37 is an explanatory diagram for showing the states of switches ofeach stage in a calibration operation of a seventh embodiment.

DETAILED DESCRIPTION

Embodiments will be described in detail. It should be noted that thesame reference numerals are given to constitutional elements having thesame functions in all the drawings for explaining the embodiments of theinvention, and the explanations thereof will not be repeated.

[First Embodiment] AD Convertor that Performs Calibration Using anAverage Value Obtained by Removing the Maximum Value and the MinimumValue

FIG. 3 is a block diagram for showing a configuration example of asignal processing circuit 10 according to a first embodiment. The signalprocessing circuit 10 includes an AD convertor (ADC) 1, an errorcorrection logic (ECL) 2, a calibration circuit 3, and a control circuit4. The ADC 1 converts the voltage value of an input analog signal VINinto a digital value ADC_RAW on the basis of a standard voltage VCM andmaximum/minimum reference voltages VREFP/VREFN to be output to the ECL2. The ADC_RAW that is a conversion result of the ADC 1 includes aredundant bit, and the ECL 2 corrects conversion errors using theredundancy to be output as an ADC_ECL. The calibration circuit 3includes a correction circuit 7, a correction value operation circuit 6,and an averaging circuit 5, and calculates and holds a correction value.The signal processing circuit 10 performs a calibration operation thatobtains a correction value before starting a normal operation. Thecontrol circuit 4 controls these operations. The correction circuit 7corrects the ADC_ECL using the correction value obtained by thecalibration to be output as a conversion output ADOUT in the normaloperation.

It should be noted that the lines shown in the drawing are mounted as anarbitrary number of signal lines. However, the number is not specifiedin the drawing, and so-called vector notation is omitted. The sameapplies to the other drawings in the application.

In the calibration operation, the ADC 1 outputs a conversion resultcorresponding to a predetermined analog signal, and the correction valueoperation circuit 6 calculates a correction value on the basis of theconversion result. A plurality of correction values can be obtained byrepeating the operation a plurality of times. The averaging circuit 5calculates the average value of the correction values to obtain thefinal correction value. The final correction value is simply referred toas a “correction value”, and each of the correction values repeatedlycalculated is also referred to as a “correction value”. However, ifthere is a possibility of confusion, each correction value is referredto as an “elemental correction value”. The averaging circuit 5 removesthe maximum value and the minimum value from the elemental correctionvalues, and calculates the average value of the remaining elementalcorrection values as the final correction value. The calculatedcorrection value may be held by the averaging circuit 5, the correctioncircuit 7, or any other circuit in the calibration circuit 3, or may beheld outside the calibration circuit 3.

The configuration of the signal processing circuit 10 shown in FIG. 3shows an example in which the ADC 1 is a sequential comparison-type ADCthat outputs a conversion result including a redundant bit, and thecorrection circuit 7 corrects the linearity of the ADC 1 due tomanufacturing variation using the correction value obtained by thecalibration. For example, offset as one of the other characteristics ofthe ADC 1 may be corrected by the calibration, or an ADC of a differentsystem may be corrected by the calibration. In this case, an output fromthe correction circuit 7 need not be input into the correction valueoperation circuit 6 as shown in the drawing, but another signal may beadded or the output may be changed to another signal. Further, the ECL 2may be omitted. In addition, the correction circuit 7 may be changed soas to supply a correction signal that can adjust the characteristics ofthe ADC 1 in an analog or digital manner to the ADC 1 instead ofcorrecting the conversion result of the ADC 1 in a digital operationprocess.

Thus, the essential requirement needed for the signal processing circuit10 is as follows. Namely, the analog to digital convertor (signalprocessing circuit) 10 includes the ADC 1 that converts the analog valueVIN of an input signal into a digital value to output the convertedvalue, and an averaging circuit 5 that calculates a correction value ina calibration operation, and corrects the converted value using thecorrection value to output the converted value ADOUT after thecorrection. The analog to digital convertor (signal processing circuit)10 is operated in the calibration operation as follows. The signalprocessing circuit 10 supplies the elemental correction value on thebasis of the converted value by the ADC 1 corresponding to apredetermined analog value to the averaging circuit 5 a plurality oftimes, and the averaging circuit 5 calculates the correction value bycalculating the average value of the remaining elemental correctionvalues obtained by removing, at least, the maximum value and the minimumvalue from the elemental correction values supplied a plurality oftimes.

Accordingly, even in the case where the converted value of the ADC 1includes a large error due to unexpected external noise or the like inthe period of the calibration operation, the correction value can becalculated by excluding the effect.

Here, the averaging circuit 5 may average the remaining elementalcorrection values obtained by removing one maximum value and one minimumvalue from the elemental correction values, or the remaining elementalcorrection values obtained by removing some large elemental correctionvalues including the maximum value and some small elemental correctionvalues including the minimum value. Further, a plurality of finalcorrection values or a plurality of types of final correction valuesused for the correction in the normal operation by the correctioncircuit 7 may be provided in accordance with a target of thecalibration, which will be described later in detail in the latter partof the first embodiment and in the other embodiments.

The signal processing circuit 10 is formed on a single semiconductorsubstrate such as silicon using, for example, a manufacturing techniqueof a well-known CMOS (Complementary Metal-Oxide-Semiconductor fieldeffect transistor) LSI (Large Scale Integrated circuit), but is notlimited to this.

FIG. 4 is a configuration diagram for showing an example of asemiconductor device 100 on which the signal processing circuit 10 ismounted. The semiconductor device 100 is, for example, an LSI formed ona single semiconductor substrate such as silicon, and includes, inaddition to the signal processing circuit 10, an input/output circuit(IO) 90, a digital signal processing circuit 92, a peripheral analogcircuit 93, a peripheral digital circuit 94, a CPU (Central ProcessingUnit) 95, and a memory 96. The analog input signal VIN is input to thesignal processing circuit 10 from the outside of the semiconductordevice 100 through the IO 90, and the output ADOUT of the signalprocessing circuit 10 is supplied to the digital signal processingcircuit 92 to be input for signal processing in the subsequent stage.

FIG. 5 is a configuration diagram for showing another example of thesemiconductor device 100 on which the signal processing circuit 10 ismounted. The digital signal processing circuit 92 is changed to asubsequent-stage signal processing circuit 92, and the semiconductordevice 100 further includes a preceding-stage signal processing circuit91. The VIN is input to the signal processing circuit 10 not from theoutside of the semiconductor device 100 but as a signal processed by thepreceding-stage signal processing circuit 91. The other configurationsare the same as those in FIG. 4, and thus the explanations thereof willbe omitted.

In the latter part of the first embodiment and in second to sixthembodiments, an ADC 1 of a sequential comparison system will bedescribed.

FIG. 6 is a block diagram for showing a configuration example of the ADC1 of a sequential comparison system. The ADC 1 shown in the drawing is a12-bit sequential comparison-type A/D convertor, and is configured usinga 6-bit capacitance DAC circuit 11 including one redundant bit, a 7-bitresistance DAC circuit 13, a comparator circuit 12, and a sequentialcomparison logic circuit 14. The ADC 1 has terminals for the inputsignal VIN, the standard voltage VCM, the output signal ADC_RAW[12:0],and the reference voltages VREFP and VREFN. The outputs of thecapacitance DAC circuit 11 on the P side and the N side are coupled to anegative input terminal and a positive input terminal of the comparatorcircuit 12, respectively. In this case, “[12:0]” represents a 13-bitdigital signal. The same applies to “[X:Y]” notation in thespecification. However, since the ADC_RAW[12:0] includes one redundantbit, and thus the amount of information is less than 13 gradation bits.The reference voltages VREFP and VREFN are analog voltages correspondingto the maximum value and the minimum value, and generally serve as apower supply potential and a ground potential, respectively, in manycases. Further, the standard voltage VCM has a potential close to(VREFP+VREFN)/2 in many cases. The configuration shown in FIG. 6 ismerely an example set for the following description, and the resolution,the resolution ratio of the capacitance DAC circuit 11 to the resistanceDAC circuit 12, and the position of the redundant bit can beappropriately changed.

FIG. 7 is a circuit diagram for showing a configuration example of thecapacitance DAC circuit 11 mounted in the sequential comparison-type ADconversion circuit 1. The 7-bit resistance DAC circuit 13 and thecomparator circuit 12 to be coupled are also shown in the drawing. Thecapacitance DAC circuit 11 has 6 bits including one redundant bit, andhas, on the p side, six capacitors Cp16, Cp8, Cp4, Cp2, Cp1, and Cp_redand six switches Sp16, Sp8, Sp4, Sp2, Sp1, and Sp_red corresponding to 6bits. As similar to the above, the capacitance DAC circuit 11 has sixcapacitors Cn16, Cn8, Cn4, Cn2, Cn1, and Cn_red and six switches Sn16,Sn8, Sn4, Sn2, Sn1, and Sn_red on the n side. The capacitance DACcircuit 11 further has two switches Sp_shunt and Sn_shunt, a capacitorCp_rdac coupled to the resistance DAC circuit 13 on the p side, and acapacitor Cn_rdac coupled to a resistive element R on the n side. Theresistance DAC circuit 13 applies a voltage between the VREFP and theVREFN to the capacitor Cp_rdac on the basis of a 7-bit digital value tobe set, so that the effective capacitance value is adjusted in a rangewhere the value equal to the capacitor Cp1 is set as the maximum value.Accordingly, the capacitance DAC circuit 11 has seven capacitors having½ to 1/128 of the capacitance value of the capacitor Cp1, and thus has afunction equivalent to a capacitance DAC circuit having six capacitorsCp16, Cp8, Cp4, Cp2, Cp1, and Cp_red.

One terminal of each of the capacitors Cp16, Cp8, Cp4, Cp2, Cp1, Cp_red,and Cp_rdac on the p side is coupled and serves as an output on the pside of the capacitance DAC circuit 11. The other terminals of thecapacitors Cp16, Cp8, Cp4, Cp2, Cp1, and Cp_red on the p side arecoupled to the six switches Sp16, Sp8, Sp4, Sp2, Sp1, and Sp_red,respectively. Each switch has three inputs. One input is for the inputsignal VIN, and the other two inputs are for the reference voltagesVREFP and VREFN. The other end of the Cp_rdac is coupled to the outputof the resistance DAC circuit 13. Further, the output of the capacitanceDAC circuit 11 on the P side is coupled to the standard voltage VCMthrough the switch Sp_shunt.

One terminal of each of the capacitors Cn16, Cn8, Cn4, Cn2, Cn1, Cn_red,and Cn_rdac on the n side is coupled and serves as an output on the Nside of the capacitance DAC circuit 11. The other terminals of thecapacitors Cn16, Cn8, Cn4, Cn2, Cn1, and Cn_red on the n side arecoupled to the six switches Sn16, Sn8, Sn4, Sn2, Sn1, and Sn_red,respectively. Each switch has three inputs. One input is for thestandard voltage VCM, and the other two inputs are for the referencevoltages VREFP and VREFN. The other end of the Cn_rdac is coupled to theVREFP through the resistor R. Further, the output of the capacitance DACcircuit 11 on the N side is coupled to the standard voltage VCM throughthe switch Sn_shunt.

The design value of each capacitor configuring the capacitance DACcircuit 11 will be described. When a unit capacitor is represented asCunit, each of the Cp16 and the Cn16 has a capacitance value obtained bycoupling 16 unit capacitors Cunit in parallel as the design value, whichcorresponds to the most significant bit ADC_RAW[12] of the conversionoutput of the ADC 1. As similar to the above, each of the Cp8 and theCn8 has a capacitance value obtained by coupling 8 unit capacitors Cunitin parallel as the design value, which corresponds to ADC_RAW[11]. Eachof the Cp4 and the Cn4 has a capacitance value obtained by coupling 4unit capacitors Cunit in parallel as the design value, which correspondsto ADC_RAW[10]. Each of the Cp2 and the Cn2 has a capacitance valueobtained by coupling 2 unit capacitors Cunit in parallel as the designvalue, which corresponds to ADC_RAW[9]. Further, each of the Cp1 and theCn1 has a capacitance value same as the unit capacitor Cunit as thedesign value, which corresponds to ADC_RAW[8]. The design value of eachof the Cp_red, the Cn_red, the Cp_rdac, and the Cn_rdac is the same asthe capacitance value of the unit capacitor Cunit. Since the designvalue of each of the Cp_red and the Cn_red is the same as thecapacitance value of the unit capacitor Cunit, the correspondingADC_RAW[7] has the same weight as the ADC_RAW[8], and functions as aredundant bit. Namely, it is possible to correct an error less than theweight same as the ADC_RAW[8] generated at each bit not less than theADC_RAW[8]. The Cp_red and the Cn_red are referred to as redundantcapacitors, and may be arbitrarily arranged at any bit positions.

FIG. 6 will be described again.

The outputs of the capacitance DAC circuit 11 on the P side and the Nside are coupled to the N side and the P side of the comparator circuit12, respectively. The resistance DAC circuit 13 is, for example, a D/Aconvertor circuit configured using resistive elements and switches, andcan be configured using an R-2R circuit, a resistance ladder circuit,and the like. When the input voltage on the P side is higher than thaton the N side, the comparator circuit 12 outputs a high level “H”. Whenthe input voltage on the P side is lower than that on the N side, thecomparator circuit 12 outputs a low level “L”. The sequential comparisonlogic circuit 14 is a circuit that supplies a control signal to eachcircuit in the ADC 1 to perform a sequential comparison operation. Thecontrol signals include a comparison start signal, a sampling startsignal, and the like, and control the capacitance DAC circuit 11, theresistance DAC circuit 13, and the comparator circuit 12 in response toan output from the comparator circuit 12.

FIG. 3 in which the ADC 1 is the 12-bit sequential comparison-type A/Dconvertor will be described again. The error correction logic (ECL) 2 isa circuit that converts the 13-bit signal ADC_RAW[12:0] including theredundant bit into a 12-bit signal ADC_ECL[11:0] including no redundantbit.

Each of the constitutional elements of the calibration circuit 3 will bedescribed using an example of a circuit that corrects the capacitancemismatch of the capacitor Cp16 corresponding to the most significant bitADC_RAW[12]. In this case, the capacitance mismatch means relativemanufacturing variation between the Cp16 and the low-orderCp8+Cp4+Cp2+Cp1+Cp_rdac. It is effective in the calibration of thelinearity of the ADC 1 to minimize a relative error rather than tocorrect an absolute error from the above-described design value of theCp16. Accordingly, the calibration circuit 3 calculates a differencebetween the Cp16 and the Cp8+Cp4+Cp2+Cp1+Cp_rdac as a correction value.A configuration example of each constitutional element of thecalibration circuit 3 to realize such calibration will be describedbelow in detail.

FIG. 8 is a block diagram for showing a configuration example of thecorrection circuit 7. The correction circuit 7 is configured using amultiplexer 35 and an adder circuit 36. After a correction valuee11[11:0] is determined, ADOUT[11:0]=ADC_ECL[11:0]+e11[11:0]*ADC_RAW[12]is satisfied. In this case, the ADOUT[11:0] is an output of thecorrection circuit 7, and the corrected output e11[11:0] of the A/Dconvertor is the correction value of the most significant bit inaccordance with the amount of capacitance mismatch of the capacitorCp16. It should be noted that an input line for the ADC_RAW[12] outputfrom the ADC 1 to the correction circuit 7 is not illustrated in FIG. 3.

FIG. 9 is a block diagram for showing a configuration example of thecorrection value operation circuit 6. The correction value operationcircuit 6 is configured using a distribution circuit 38 that divides thevalue of the ADOUT[11:0] into a conversion result ADOUT_C in a samplingstate 1 and a conversion result ADOUT_S in a sampling state 2 to holdthe results, and a subtraction circuit 37 that obtains a differenceADC_SUB[11:0] between the both results. The sampling state 1 and thesampling state 2 will be described later.

FIG. 10 is a block diagram for showing a configuration example of anaveraging circuit 5 having a maximum value/minimum value removalfunction. The averaging circuit 5 is configured using a maximumvalue/minimum value removal circuit 30, an accumulation circuit 31, adivision circuit 32, and a register 34. The difference ADC_SUB[11:0]from the correction value operation circuit 6 is repeatedly input aplurality of times (N times). In this case, N is a given integer. When alarge value is given to N, much time is required for the calibration.However, an effect of random noise can be reduced. The maximumvalue/minimum value removal circuit 30 removes one maximum value and oneminimum value from N differences ADC_SUB[11:0], and the remaining N−2differences are output to the accumulation circuit 31. The accumulationcircuit 31 accumulates the input N−2 differences ADC_SUB[11:0] to obtaina total value, and outputs the same to the division circuit 32. Thedivision circuit 32 divides the input total value by N−2 to calculatethe average value of the correction values with the maximum value andthe minimum value removed, and the average value is held by the register34. The register 34 holds the final correction value e11[11:0].

FIG. 11 is a block diagram for showing a configuration example of themaximum value/minimum value removal circuit 30. The ADC_SUB[11:0] andcontrol signals RES and COMP are input into the maximum value/minimumvalue removal circuit 30, and the maximum value/minimum value removalcircuit 30 is configured using two registers (REG0 and REG1) 40 and 41,two comparators 42 and 43, and a multiplexer 44 having four inputs andone output. The two comparators 42 and 43 compare the values stored inthe two registers (REG0 and REG1) 40 and 41 with the input ADC_SUB[11:0]to output comparison results CMP_RES[1:0]. A comparison resultCMP_RES[1] shows that the input ADC_SUB[11:0] is larger than the valuestored in the register (REG1) 41, and a comparison result CMP_RES[0]shows that the input ADC_SUB[11:0] is smaller than the value stored inthe register (REG0) 40. When the comparison result CMP_RES[1] isasserted, the register (REG1) 41 reads and stores the ADC_SUB[11:0].When the comparison result CMP_RES[0] is asserted, the register (REG0)40 reads and stores the ADC_SUB[11:0]. In the multiplexer 44, input arethe ADC_SUB[11:0], the value stored in the register (REG1) 41, the valuestored in the register (REG0) 40, and 0, which are selected inaccordance with the comparison result CMP_RES[1:0] to be output to theaccumulation circuit 31.

During the calibration operation, the maximum value of the differencesADC_SUB[11:0] input from the beginning of the accumulation to the timeis stored in the register (REG1) 41, and the minimum value is similarlystored in the register (REG0) 40. If the ADC_SUB[11:0] input at the timeis larger than the value stored in the register (REG1) 41, theCMP_RES[1:0] is 10, and the value stored in the register (REG1) 41,namely, the maximum value up to the time is output to the accumulationcircuit 31. In addition, a new ADC_SUB[11:0] at the time, namely, a newmaximum value is stored in the register (REG1) 41. If the ADC_SUB[11:0]input at the time is smaller than the value stored in the register(REG0) 40, the CMP_RES[1:0] is 01, and the value stored in the register(REG0) 40, namely, the minimum value up to the time is output to theaccumulation circuit 31. In addition, a new ADC_SUB[11:0] at the time,namely, a new minimum value is stored in the register (REG0) 40. In thecase where the ADC_SUB[11:0] input at the time is between the valuestored in the register (REG0) 40 and the value stored in the register(REG1) 41, the ADC_SUB[11:0] is output to the accumulation circuit 31 asit is, and the values of the registers (REG0 and REG1) 40 and 41 are notupdated.

One maximum value and one minimum value among the input N differencesADC_SUB[11:0] are removed by such an operation, and the remaining N−2differences are output to the accumulation circuit 31.

It should be noted that since initial values are stored in the registers(REG0 and REG1) 40 and 41 when the accumulation is started, thefollowing control is further needed. The CMP_RES[1:0] is forcibly set to11 by the control signal COMP in two rounds immediately after thebeginning of the accumulation. The first ADC_SUB[11:0] immediately afterthe beginning of the accumulation is forcibly stored in each of theregisters (REG0 and REG1) 40 and 41 by the control signal RES.

FIG. 12 is a block diagram for showing another configuration example ofthe maximum value/minimum value removal circuit 30. The maximumvalue/minimum value removal circuit 30 is configured using N registers(REG_1 to REG_N) 39_1 to 39_N storing N input signals ADC_SUB[11:0], anda search removal circuit 45 that searches the values stored in theregisters for the maximum value and the minimum value and sequentiallyoutputs the values stored in the registers other than the maximum valueand the minimum value to the accumulation circuit 31.

The maximum value/minimum value removal circuit 30 is operated intemporally-separated phases such as a phase 1 and a phase 2 aftercompletion of the phase 1. In the phase 1, the sequentially-input valuesADC_SUB[11:0] are sequentially stored in the N registers (REG_1 toREG_N) 39_1 to 39_N. In the phase 2, the maximum value and the minimumvalue are searched for among the values stored in the N registers (REG_1to REG_N) 39_1 to 39_N, and the values stored in the registers otherthan the maximum value and the minimum value are sequentially output tothe accumulation circuit 31.

In addition to the configuration examples exemplified in FIG. 11 andFIG. 12, the maximum value/minimum value removal circuit 30 can employvarious configurations without departing from the scope thereof.

In the case where the capacitance mismatch occurs among the capacitorsconfiguring the capacitance DAC circuit 11 in the ADC 1, the linearityof the A/D conversion result is deteriorated.

FIG. 13 is an explanatory diagram for showing an operation example ofthe calibration. The capacitance mismatch of the capacitor Cp16corresponding to the most significant bit is corrected by thecalibration operation shown in the first embodiment. FIG. 13 showsgraphs of A/D conversion characteristics before and after thecalibration in which the horizontal axis represents an input analogvoltage VIN and the vertical axis represents a digital conversion resultADOUT. A relation around where the input voltage VIN is equal to thestandard voltage VCM (VIN=VCM) is enlarged in each graph. The graphsshow the A/D conversion characteristics of three cases in which the Cp16is in an ideal state (Cp16=Cp16_ideal), the capacitance value of theCp16 is biased to the large side due to mismatch (Cp16>Cp16_ideal), andthe capacitance value of the Cp16 is biased to the small side(Cp16<Cp16_ideal). In this case, “the Cp16 is in an ideal state” doesnot mean that the capacitance value of the Cp16 is equal to the designvalue (absolute accuracy), but means that the capacitance value of theCp16 is ideal from the viewpoint of consistency (relative accuracy) ofthe capacitance values of the other capacitors. For example, it is idealthat the capacitance value of the Cp16 is exactly twice the capacitancevalue of the Cp8, exactly four times the capacitance value of the Cp4,exactly eight times the capacitance value of the Cp2, and exactlysixteen times each capacitance value of the Cp1, Cp_red, and Cp_rdac.Accordingly, “Cp16_ideal” merely means the ideal state in a symbolicmanner, and a concrete number is not defined.

The most significant bit of the A/D conversion result is switched aroundVIN=VCM. Namely, when the VIN is lower than the VCM, the mostsignificant bit ADC_RAW[12] is 0. When the VIN is higher than the VCM,the most significant bit ADC_RAW[12] is 1. When the VIN is lower thanthe VCM, the Cp16 does not contribute to the conversion. When the VIN ishigher than the VCM, the Cp16 contributes to the conversion. Thus, ifthe mismatch of the capacitance value of the Cp16 occurs, the conversionresult is deviated from the ideal linear characteristics, and showsstepped characteristics. Namely, when the capacitance value of the Cp16is biased to the large side (Cp16>Cp16_ideal), the conversion result isbiased to the small side. When the capacitance value of the Cp16 isbiased to the small side (Cp16<Cp16_ideal), the conversion result isbiased to the large side. In the first embodiment, the steps areeliminated around VIN=VCM where the ADCOUT[11] is switched from 0 to 1by adding the correction value e11[11:0] to the ADC_ECL[11:0] when theADC_RAW[12]=1. In the lower part of FIG. 13, the dashed lines representthe characteristics (ADC_ECL[11:0]) before the correction, the arrowsrepresent the correction value e11[11:0], the solid lines represent thecharacteristics ADOUT[11:0] after the correction.

The calibration operation will be described in more detail.

FIG. 16 is a timing chart for showing an operation example in which thecorrection value of the capacitor of the most significant bit in thecapacitance DAC circuit 11 is calculated in the calibration operation.The circuit diagrams of FIG. 14 and FIG. 15 show an internal switchcoupling state of the capacitance DAC circuit 11 and an output statefrom the resistance DAC circuit 13 in a sampling operation period at thetime of the calibration operation when the correction value e11[11:0] isobtained for the capacitance mismatch of the capacitor Cp16corresponding to the most significant bit.

FIG. 14 shows the states of the switches and an output state from theresistance DAC circuit 13 in the sampling state (state 1) of thecapacitor Cp16 corresponding to the most significant bit in thecapacitance DAC circuit 11. The outputs of the capacitance DAC circuit11 on the P side and the N side are coupled to the standard voltage VCMthrough the switches Sp_shunt and Sn_shunt, respectively. The capacitorCp16 is coupled to the VREFP through the switch Sp16, and the capacitorsCp8, Cp4, Cp2, Cp1, and Cp_red are coupled to the VREFN through theswitches Sp8, Sp4, Sp2, Sp1, and Sp_red, respectively. The resistanceDAC circuit 13 outputs the VREFN, which is accordingly equivalent to thestate in which the capacitor Cp_rdac is coupled to the VREFN.

FIG. 15 shows the states of the switches and an output state from theresistance DAC circuit 13 in the sampling state (state 2) of thecapacitors (Cp8+Cp4+Cp2+Cp1+Cp_rdac) lower than the most significant bitin the capacitance DAC circuit 11. As similar to the above-describedstate 1, the outputs of the capacitance DAC circuit 11 on the P side andthe N side are coupled to the standard voltage VCM through the switchesSp_shunt and Sn_shunt, respectively. The capacitor Cp16 is coupled tothe VREFN through the switch Sp16, and the capacitors Cp8, Cp4, Cp2, andCp1 are coupled to the VREFP through the switches Sp8, Sp4, Sp2, andSp1, respectively. The capacitor Cp_red is coupled to the VREFN throughthe switch Sp_red as similar to the state 1. On the other hand, theresistance DAC circuit 13 outputs the VREFP, which is accordinglyequivalent to the state in which the capacitor Cp_rdac is coupled to theVREFP.

FIG. 16 will be described again.

When the calibration operation is started at Time t0, the ADC 1 performsthe conversion operation after the sampling in the state 1 is performed,and outputs a conversion result 1 at Time t1. The conversion operationis performed at Time t1 after the sampling in the state 2 is performed,and a conversion result 2 is output at Time t2. Thereafter, the firstcorrection value e11_1 is calculated between Time t2 and Time t4. Thisis the first correction value operation. In the ideal state where noiseis not present at all, a difference (the conversion result 1−theconversion result 2) obtained by subtracting the conversion result 2from the conversion result 1 corresponds to the correction valuee11[11:0]. The calculation method of the correction value usesconversion of a difference between electric charges sampled at the Cp16in the state 1 and electric charges sampled at theCp8+Cp4+Cp2+Cp1+Cp_rdac in the state 2 into the amount of mismatch ofthe Cp16. The conversion result 1 (ACDOUT_C) and the conversion result 2(ADOUT_S) are input into the subtraction circuit 37 by the distributioncircuit 38 so that a difference obtained by subtracting the conversionresult 2 from the conversion result 1 corresponds to the ADC_SUB[11:0]in the correction value operation circuit 6 (FIG. 9), and a subtractionis performed while adjusting the timing to output the value obtained bysubtracting the conversion result 2 from the conversion result 1 as theADC_SUB[11:0]. The timing is control by the control signal output fromthe control circuit 4 (FIG. 3).

As similar to the above, a correction value e11_2 is calculated byperforming the sampling in the state 1 from Time t2, the AD conversion,the sampling in the state 2, the AD conversion, and the first correctionvalue operation (the conversion result 1−the conversion result 2). Thisoperation is repeated N times in total in a pipeline manner, so that Ncorrection values e11_1 to e11_N are obtained until Time t9. The resultis input to the averaging circuit 5 to remove the maximum value and theminimum value, and the average value of the remaining N−2 correctionvalues is calculated to obtain the correction value e11 of the Cp16.

In the ideal state where noise is not present at all, the value of theADC_SUB[11:0] corresponds to the correction value e11[11:0]. However, anerror is actually included in the value of the ADC_SUB[11:0] due to theeffect of noise (elemental noise, power supply noise, noise sneaked fromother circuits, or the like). If a set of the sampling in the state 1,the AD conversion, the sampling in the state 2, the AD conversion, andthe correction value operation (the conversion result 1−the conversionresult 2) is repeated N times, N values ADC_SUB[11:0], namely, Ncorrection values e11_1 to e11_N are obtained. The averaging circuit 5performs an operation in which N−2 values ADC_SUB[11:0] obtained byremoving the maximum value and the minimum value from the N correctionvalues e11_1 to e11_N (ADC_SUB[11:0]) are accumulated, and then theaccumulated value is divided by N−2 so that the average value iscalculated to output the correction value e11[11:0].

The calibration operation for the capacitor Cp16 corresponding to themost significant bit has been described above. However, capacitorscorresponding to a plurality of bits higher than the redundant bit canbe targeted for the calibration operation.

A calibration operation in which high-order 3 bits are corrected will bedescribed. The correction value e11[11:0] of the Cp16, the correctionvalue e10[11:0] of the Cp8, and the correction value e9[11:0] of the Cp4are sequentially calculated. It is determined on the basis of theADC_RAW[12:10] whether or not each of the capacitors Cp16, Cp8, and Cp4has contributed to the AD conversion. If contributed, the correspondingcorrection value among the correction values e11[11:0], e10[11:0], ande9[11:0] is added to the conversion result ADC_ECL[11:0] to obtain aconversion output ADOUT[11:0].

FIG. 19 is a timing chart for showing an operation example in which thecorrection values e11[11:0], e10[11:0], and e9[11:0] of the capacitorsCp16, Cp8, and Cp4 from the most significant bit to the third bit in thecapacitance DAC circuit 11 are calculated in the calibration operation.The circuit diagrams of FIG. 17 and FIG. 18 show an internal switchcoupling state of the capacitance DAC circuit 11 and an output statefrom the resistance DAC circuit 13 in the sampling operation period ofthe calibration operation when the correction value e9[11:0] is obtainedfor the capacitance mismatch of the capacitor Cp4 corresponding to thethird bit from the most significant bit.

FIG. 17 shows the states of the switches and an output state from theresistance DAC circuit 13 in the sampling state (state 1) of thecapacitor Cp4 corresponding to the third bit from the most significantbit in the capacitance DAC circuit 11. The outputs of the capacitanceDAC circuit 11 on the P side and the N side are coupled to the standardvoltage VCM through the switches Sp_shunt and Sn_shunt, respectively.The capacitor Cp4 is coupled to the VREFP through the switch Sp4, andthe capacitors Cp16, Cp8, Cp2, Cp1, and Cp_red are coupled to the VREFNthrough the switches Sp16, Sp8, Sp2, Sp1, and Sp_red, respectively. Theresistance DAC circuit 13 outputs the VREFN, which is accordinglyequivalent to the state in which the capacitor Cp_rdac is coupled to theVREFN.

FIG. 18 shows the states of the switches and an output state from theresistance DAC circuit 13 in the sampling state (state 2) of thecapacitors (Cp2+Cp1+Cp_rdac) lower than the third bit from the mostsignificant bit in the capacitance DAC circuit 11. As similar to theabove-described state 1, the outputs of the capacitance DAC circuit 11on the P side and the N side are coupled to the standard voltage VCMthrough the switches Sp_shunt and Sn_shunt, respectively. The capacitorsCp16, Cp8, and Cp4 are coupled to the VREFN through the switches Sp16,Sp8, and Sp4, respectively, and the capacitors Cp2 and Cp1 are coupledto the VREFP through the switches Sp2 and Sp1, respectively. Thecapacitor Cp_red is coupled to the VREFN through the switch Sp_red assimilar to the state 1. On the other hand, the resistance DAC circuit 13outputs the VREFP, which is accordingly equivalent to the state in whichthe capacitor Cp_rdac is coupled to the VREFP.

Although not shown in the drawing, the operation of calculating thecorrection value by performing the sampling in the state 1, the ADconversion, the sampling in the state 2, the AD conversion, and thecorrection value operation (the conversion result 1−the conversionresult 2) according to the sequence similar to FIG. 16 is repeated Ntimes in total in a pipeline manner, so that N correction values e9_1 toe9_N are obtained. The result is input to the averaging circuit 5, andthe average value of the N−2 correction values obtained by removing themaximum value and the minimum value is calculated to obtain thecorrection value e9 of the Cp4.

In order to calculate three correction values corresponding to threecapacitors from the most significant bit to the third bit to be used forcorrection, the calibration circuit 3 includes a correction circuit 7shown in FIG. 20 instead of the correction circuit shown in FIG. 8, andincludes an averaging circuit 5 shown in FIG. 21 instead of theaveraging circuit shown in FIG. 10.

FIG. 21 is a block diagram for showing a configuration example of anaveraging circuit 5 having a maximum value/minimum value removalfunction adapted for the calibration in which the correction values ofthe capacitors from the most significant bit to the third bit in thecapacitance DAC circuit are calculated. As similar to FIG. 10, theADC_SUB[11:0] is input. The averaging circuit 5 is configured using amaximum value/minimum value removal circuit 30, an accumulation circuit31, a division circuit 32, and three registers 34_11, 34_10, and 34_9.In the above-described sequence, the maximum value and the minimum valueare removed from the N correction values e9_1 to e9_N input from theADC_SUB[11:0] by the maximum value/minimum value removal circuit 30, andthe total of the remaining N−2 correction values is calculated by theaccumulation circuit 31. The total is divided by N−2 by the divisioncircuit 32 to calculate the correction value e9 of the Cp4 as theaverage value, and the correction value e9 is stored in the register34_9.

The sampling operation and the calculation operation of the correctionvalue in the calibration operation when the correction value e10[11:0]is obtained for the capacitance mismatch of the capacitor Cp8corresponding to the second bit from the most significant bit are thesame as the above, and the calculated correction value e10[11:0] isstored in the register 34_10 of the averaging circuit 5. Further, thesampling operation and the calculation operation of the correction valuein the calibration operation when the correction value e11[11:0] isobtained for the capacitance mismatch of the capacitor Cp16corresponding to the most significant bit are the same as the operationsdescribed with reference to FIG. 14 to FIG. 16, and the calculatedcorrection value e11[11:0] is stored in the register 34_11 of theaveraging circuit 5. The accumulation circuit 31 is reset every time thecorrection values e9[11:0], e10[11:0], and e11[11:0] are calculated.

FIG. 20 is a block diagram for showing a configuration example of thecorrection circuit 7 adapted for the calibration in which the correctionvalues e11[11:0], e10[11:0], and e9[11:0] of the capacitors Cp16, Cp8,and Cp4 from the most significant bit to the third bit in thecapacitance DAC circuit 11 are calculated. The correction circuit 7 isconfigured using three multiplexers 35_11, 35_10, and 35_10corresponding to three correction values and an adder circuit 36. Whenthe ADC_RAW[12]=1, it is determined that the capacitor Cp16 hascontributed to the A/D conversion, and the corresponding correctionvalue e11[11:0] is added to the ADC_ECL[11:0]. As similar to the above,when the ADC_RAW[11]=1, it is determined that the capacitor Cp8 hascontributed to the A/D conversion. When the ADC_RAW[10]=1, it isdetermined that the capacitor Cp4 has contributed to the A/D conversion.Then, the corresponding correction values e10[11:0] and e9[11:0] areadded to the ADC_ECL[11:0]. Namely, the equation ofADOUT[11:0]=ADC_ECL[11:0]+e11[11:0]*ADC_RAW[12]+e10[11:0]*ADC_RAW[11]+e9[11:0]*ADC_RAW[10]is satisfied.

In the above description, the number of correction values before theaveraging that are obtained when calculating the correction valuese11[11:0], e10[11:0], and e9[11:0] is N in all the cases. However,different numbers can be set depending on the correction value. On theother hand, if the same number is set, it is only necessary for theaveraging circuit 5 to repeat the same operation, and thus theconfiguration and the control can be simplified.

FIG. 19 will be described again.

When the calibration operation is started at Time t0, the correctionvalue e9[11:0] of the Cp4 is calculated first. A set of the sampling inthe state 1, the AD conversion, the sampling in the state 2, the ADconversion, and the correction value operation (the conversion result1−the conversion result 2) is repeated, and the remaining valuesobtained by removing the maximum value and the minimum value areaveraged, so that the correction value e9[11:0] of the Cp4 can beobtained at Time t1. Next, the correction value e10[11:0] of the Cp8 iscalculated from Time t1. As similar to the above, a set of the samplingin the state 1, the AD conversion, the sampling in the state 2, the ADconversion, and the correction value operation (the conversion result1−the conversion result 2) is repeated, and the remaining valuesobtained by removing the maximum value and the minimum value areaveraged, so that the correction value e10[11:0] of the Cp8 can beobtained at Time t2. Finally, the correction value e11[11:0] of the Cp16is calculated from Time t2. As similar to the above, a set of thesampling in the state 1, the AD conversion, the sampling in the state 2,the AD conversion, and the correction value operation (the conversionresult 1−the conversion result 2) is repeated, and the remaining valuesobtained by removing the maximum value and the minimum value areaveraged, so that the correction value e11[11:0] of the Cp16 can beobtained at Time t3. From Time t4, the normal operation can be performedby operating the correction circuit 7 described with reference to FIG.20.

Accordingly, the calibration for not only the most significant bit, butalso the capacitors corresponding to the bits lower than the mostsignificant bit can be performed. According to the operation principle,the bits higher than the redundant bit can be corrected.

As described above, the first embodiment is characterized in that themaximum value/minimum value removal circuit 30 is provided in theaveraging circuit 5 in the calibration circuit 3. Even in the case whereone of the correction value operation results includes a value largelydeviated from the value that is supposed to be the correction value dueto unexpected external noise such as noise of the power supply, theground, or the reference voltage, or noise sneaked from the inside oroutside of the semiconductor device during the correction valueoperation period of the calibration as shown in FIG. 2B, the deviatedvalue can be removed from the averaging targets by providing the maximumvalue/minimum value removal circuit 30. Accordingly, a value close tothe value that is supposed to be the correction value is likely to beselected, and the accuracy of the calibration is improved.

On the other hand, even in the case of an averaging circuit having nosuch characteristics, the effect of the unexpected external noise can beremoved in principle by largely increasing the number of times ofaveraging. However, a period of time that can be set as a startingperiod of time in a system is generally limited in actual use. Namely, aperiod of time that can be assigned to the calibration is limited. Inorder to eliminate the deviation of the correction value due to theeffect of the unexpected external noise that is difficult to predict inthe design stage by increasing the number of times of averaging, thenumber of times of averaging is needlessly increased. The considerationfor the effect due to the unexpected noise can be eliminated whensetting the number of times of averaging by providing thecharacteristics, and the optimum number of times of averaging can beset. Namely, it is only necessary to set the number of times ofaveraging that can remove the effect of random noise such as elementalnoise.

It should be noted that the bit positions where the redundant capacitorsCp_red and Cp_rdac are mounted can be arbitrarily set as describedabove. However, the calibration can be performed for the capacitorshigher than the bit positions where the redundant capacitors Cp_red andCp_rdac are mounted. Further, the calibration operation is performedfrom the low-order bit to the most significant bit in order as describedin the embodiment. An error in a low-order bit is corrected by thecalibration for the bit, and the calibration for high-order bits can bemore accurately performed. It should be noted that the number of bitsfor which the calibration is actually executed can be arbitrarily set.

[Second Embodiment] Registers Holding the Maximum Value and the MinimumValue

A second embodiment shows another configuration example for adding afunction of removing the maximum value and the minimum value to anaveraging circuit 5 in a calibration circuit 3.

FIG. 22 is a block diagram for showing a configuration example of anaveraging circuit 5 having a maximum value/minimum value removalfunction according to the second embodiment. As similar to the averagingcircuit 5 shown in FIG. 10, the averaging circuit 5 includes theaccumulation circuit 31 and the register 34. On the other hand, theaveraging circuit 5 includes a register 41 holding the maximum value, aregister 40 holding the minimum value, and a subtraction circuit 47instead of providing the maximum value/minimum value removal circuit 30,and includes an R-bit right shift circuit 33 (R is an integer determineddepending on N) instead of the division circuit 32.

After the reset is cancelled, the accumulation circuit 31 calculates thetotal value of N correction values e11_1 to e11_N input to theADC_SUB[11:0]. The register 41 sequentially compares the N correctionvalues e11_1 to e11_N in parallel to finally hold the maximum value. Theregister 40 also sequentially compares the N correction values e11_1 toe11_N in parallel to finally hold the minimum value. The subtractioncircuit 47 subtracts the maximum value input from the register 41 to aterminal B and the minimum value input from the register 40 to aterminal C from the total value of the N correction values e11_1 toe11_N input to a terminal A, and outputs the result. In this case, ifthe number N of correction values before the averaging is set to “theR-th power of 2+2”, the division circuit 32 using N−2 can be replaced bythe R-bit right shift circuit 33. The R-bit right shift circuit 33 doesnot need active circuits such as transistors as long as R is a fixedvalue, and can be mounted only by changing wirings. The calculatedaverage value is held by the register 34 as the correction valuee11[11:0].

The other configurations and operations are the same as those in thefirst embodiment, and thus the explanations thereof will be omitted. Inthis case, the second embodiment can be changed to an embodiment inwhich a calibration operation is performed for capacitors correspondingto a plurality of bits. In the averaging circuit 5 shown in FIG. 22, theregister 34 is changed to a plurality of registers (for example, theregisters 34_11, 34_10, and 34_9) storing a plurality of correctionvalues as similar to FIG. 21, and the accumulation circuit 31, themaximum value register 41, and the minimum value holding register 40 areconfigured and controlled to be reset every time the correction valuesare calculated and stored in the registers.

Accordingly, it is not necessary to mount the maximum value/minimumvalue removal circuit 30 as described with reference to FIG. 11 and FIG.12, and the circuit scale can be reduced. In addition, the control canbe simplified.

Further, the circuit scale necessary for a division can be largelyreduced by replacing the division circuit 32 with the R-bit right shiftcircuit 33. It should be noted that if the number N of correction valuesbefore the averaging is set to “the R-th power of 2+2”, the divisioncircuit 32 using N−2 can be replaced by the R-bit right shift circuit 33in the first embodiment.

[Third Embodiment] Registers Holding X Large Values and X Small Values

In the first and second embodiments, one maximum value and one minimumvalue are removed from N correction values before averaging, and theremaining N−2 correction values are averaged to obtain the finalcorrection value. Accordingly, values deviated from the value supposedto be the correction value due to the effect of unexpected noise areremoved to calculate a more accurate correction value. On the contrary,X large correction values including the maximum value and X smallcorrection values including the minimum value are removed from Ncorrection values before averaging, and the remaining N−2× correctionvalues are averaged to obtain the final correction value in theembodiment. In this case, the number of large correction values and thenumber of small correction values to be removed may be different fromeach other.

Such expansion of functions can be realized by improving the averagingcircuit 5 shown in FIG. 22.

FIG. 23 is a block diagram for showing a configuration example of anaveraging circuit 5 having a maximum value/minimum value removalfunction according to the third embodiment. As similar to the averagingcircuit 5 shown in FIG. 22, the averaging circuit 5 includes theaccumulation circuit 31, the subtraction circuit 47, the R-bit rightshift circuit 33, and the register 34. On the other hand, the register41 holding the maximum value and the register 40 holding the minimumvalue are expanded to a large value holding register 41 holding X largevalues and a small value holding register 40 holding X small values,respectively, and an adder circuit 46 is further provided.

After the reset is cancelled, the accumulation circuit 31 calculates thetotal value of N correction values e11_1 to e11_N input to theADC_SUB[11:0]. The register 41 sequentially compares the N correctionvalues e11_1 to e11_N in parallel to hold X large correction valuesincluding the maximum value. The register 40 also sequentially comparesthe N correction values e11_1 to e11_N in parallel to hold X smallcorrection values including the minimum value. The adder circuit 46 addsthe X large correction values held by the register 41 to the X smallcorrection values held by the register 40, and supplies the total to thesubtraction circuit 47. The subtraction circuit 47 subtracts the totalvalue of the X large correction values and the X small correction valuesinput to a terminal B from the total value of the N correction valuese11_1 to e11_N input to a terminal A, and outputs the result. In thiscase, if the number N of correction values before the averaging is setto “the R-th power of 2+2×”, the R-bit right shift circuit 33 can beused. The calculated average value is held by the register 34 as thecorrection value e11[11:0].

The other configurations and operations are the same as those in thefirst and second embodiments, and thus the explanations thereof will beomitted. In this case, the third embodiment can be changed to anembodiment in which a calibration operation is performed for capacitorscorresponding to a plurality of bits. In the averaging circuit 5 shownin FIG. 23, the register 34 is changed to a plurality of registers (forexample, the registers 34_11, 34_10, and 34_9) storing a plurality ofcorrection values as similar to FIG. 21, and the accumulation circuit31, the large value holding register 41, and the small value holdingregister 40 are configured and controlled to be reset every time thecorrection values are calculated and stored in the registers.

As described above, even if the correction value operation resultsinclude a plurality of largely-deviated results due to unexpectedexternal noise generated a plurality of times, the largely-deviatedresults can be removed by expanding the large value holding register 41and the small value holding register 40. The configuration is moreeffective from the viewpoint of removing the effect of the unexpectedexternal noise. Therefore, it is possible to withstand calibration in anoisy circumstance as compared to the first and second embodiments.Further, there may be a circumstance in which large unexpected noise isperiodically, regularly, and relatively-frequently generated so that thecorrection value operation results are deviated by a specific amount. Inthis case, it is impossible to remove the effect of the noise inprinciple by simply increasing the number of times of averaging.However, the effect can be removed by providing the characteristics ofthe embodiment.

[Fourth Embodiment] Limiter

In the case where external noise more than expected is furthergenerated, the deviation of the correction value can be reduced byadding a limiter to the averaging circuit 5 of each of the first tothird embodiments. FIGS. 24 to 26 are block diagrams for showing variousconfiguration examples of an averaging circuit 5 to which a limiter of afourth embodiment is added. In this case, the limiter is a circuit thatlimits an output when an input value is deviated from a setting range.For example, when an input value is larger than a predetermined upperlimit, the value is replaced by the upper limit. When an input value issmaller than a predetermined lower limit, the value is replaced by thelower limit.

FIG. 24 is a block diagram for showing a first configuration example ofthe averaging circuit 5. As similar to the averaging circuit shown inFIG. 10, the averaging circuit 5 includes the maximum value/minimumvalue removal circuit 30, the accumulation circuit 31, the divisioncircuit 32, and the register 34, and further includes a limiter 50_2 onthe preceding stage of the register 34 holding the value of thecorrection value e11[11:0]. When an output of the division circuit(÷(N−2) computing unit) 32 is deviated from the setting range of thelimiter 50_2, the limiter 50_2 can limit the correction value e11[11:0].

FIG. 25 is a block diagram for showing a second configuration example ofthe averaging circuit 5. As similar to the averaging circuit shown inFIG. 10, the averaging circuit 5 includes the maximum value/minimumvalue removal circuit 30, the accumulation circuit 31, the divisioncircuit 32, and the register 34, and further includes a limiter 50_1 onthe preceding stage of the maximum value/minimum value removal circuit30. When a correction value input before averaging is deviated from thesetting range of the limiter 50_1, the limiter 50_1 can limit thecorrection value. Accordingly, a value out of an assumed range is notinput to the accumulation circuit 30, and, for example, overflow in theaccumulation circuit 30 can be prevented from being generated.

FIG. 26 is a block diagram for showing a third configuration example ofthe averaging circuit 5. As similar to the averaging circuit shown inFIG. 10, the averaging circuit 5 includes the maximum value/minimumvalue removal circuit 30, the accumulation circuit 31, the divisioncircuit 32, and the register 34, and further includes the limiter 50_1on the preceding stage of the maximum value/minimum value removalcircuit 30 and the limiter 50_2 on the preceding stage of the register34. When a correction value input before averaging is deviated from thesetting range of the limiter 50_1, the limiter 50_1 can limit thecorrection value. When an output of the division circuit (÷(N−2)computing unit) 32 is deviated from the setting range of the limiter50_2, the limiter 50_2 can limit the correction value e11[11:0].Accordingly, a value out of an assumed range is not input to theaccumulation circuit 30, and, for example, overflow in the accumulationcircuit 30 can be prevented from being generated. In addition, thecorrection value e11[11:0] to be calculated can be limited.

By adding the limiter(s) 50_1 and/or 50_2 to the averaging circuit 5 asshown in the fourth embodiment, it is possible to prevent a situation inwhich the accuracy of A/D conversion is more deteriorated in the casewhere the calibration is performed in the surrounding circumstance ofthe calibration noisier more than expected than the case where thecalibration is not performed. Even if a value largely deviated from thecorrection value to be actually obtained is calculated due to externalnoise at the time of calibration, it is possible to prevent thedeterioration of accuracy above a certain level by providing the limiter50_2 on the preceding stage of the register 34. Even if large noise thatcannot be removed in the subsequent stage is present, the effect can beeased to some extent by averaging by proving the limiter 50_1 on thepreceding stage of the maximum value/minimum value removal circuit 30,because the input value to the accumulation circuit 31 is limited.

In this case, the setting values (upper limit/lower limit) of thelimiters 50_1 and 50_2 are desirably determined in consideration ofvariation of elements to be corrected so as to prevent the correctionvalue e11[11:0] supposed to be obtained from being deviated from thesetting range of each limiter. On the other hand, in the case whereerrors due to actual manufacturing variation of capacitors are large anda correct correction value is deviated from the setting range of eachlimiter, the products are selected in a circumstance with less externalnoise at the time of product delivery, so that the selected products canbe removed from the delivery targets.

In the above description of the fourth embodiment, a calibrationoperation is performed for only the Cp16 corresponding to the mostsignificant bit. However, the fourth embodiment can be changed to anembodiment in which a calibration operation is performed for capacitorscorresponding to a plurality of bits. For example, in the averagingcircuit 5 shown in FIG. 10, the limiter 50_2 is provided in thesubsequent stage of the division circuit 32, and/or the limiter 50_1 isprovided in the preceding stage of the maximum value/minimum valueremoval circuit 30. The limiter 50_2 may be provided in the precedingstage of each of the registers 34_11, 34_10, and 34_9. Further, forexample, in the averaging circuit 5 shown in FIG. 22 according to thesecond embodiment, the register 34 is changed to the registers (forexample, the registers 34_11, 34_10, and 34_9) storing the correctionvalues as similar to FIG. 21, and the limiter 50_2 is provided in thesubsequent stage of the division circuit 32, and/or the limiter 50_1 isprovided in the preceding stage of the maximum value/minimum valueremoval circuit 30. In this case, the limiter 50_2 may be provided inthe preceding stage of each of the registers 34_11, 34_10, and 34_9.

[Fifth Embodiment] Offset Calibration

Each of the first to fourth embodiments shows an application example tocalibration for mismatch components of the capacitive elements in thecapacitance DAC circuit 11 in the sequential comparison-type A/Dconvertor. However, the present invention can be applied to calibrationfor other components. In a fifth embodiment, calibration for offseterrors of the ADC 1 that is the sequential comparison-type A/D convertorsame as that in each of the first to fourth embodiments will bedescribed.

FIG. 27 is a block diagram for showing a configuration example of an ADconvertor with an offset calibration circuit that is a signal processingcircuit 10 according to the fifth embodiment. As similar to the signalprocessing circuit shown in FIG. 3, the signal processing circuit 10includes the ADC 1, the ECL 2, the calibration circuit 3, and thecontrol circuit 4, and further includes a switch SW1 on a route from theinput voltage VIN to the ADC 1, a switch SW2 on a route from thestandard voltage VCM to the ADC 1, and a switch SW3 between the inputterminals of the input voltage VIN and the standard voltage VCM of theADC 1. The control circuit 4 controls the switch SW1, the switch SW2,and the switch SW3 in addition to the ADC 1, the ECL 2, and thecalibration circuit 3.

The calibration circuit 3 is configured using the correction circuit 7and the averaging circuit 5. As shown in FIG. 28, the correction circuit5 adds an output ADC_ECL[11:0] of the error correction logic (ECL) 2 toan offset correction value e_dc[11:0] to be output as a conversionoutput ADOUT[11:0]. As shown in FIG. 29, the averaging circuit 5 is acircuit similar to the averaging circuit of FIG. 10, and removes themaximum value and the minimum value from the outputs ADC_ECL[11:0]during the calibration period to be accumulated by the accumulationcircuit 31. Thereafter, the averaging circuit 5 stores into the register34 the average value obtained by being divided by N−2 by the divisioncircuit 32 as the offset correction value e_dc[11:0]. The averagingcircuit 5 may be changed to a circuit similar to the averaging circuitshown in each of FIG. 22 to FIG. 26.

An offset calibration operation will be described.

FIG. 31 is a timing chart for showing an operation example of the offsetcalibration.

The circuit diagram of FIG. 30 shows the states of the switches SW1,SW2, and SW3, the states of the switches in the capacitance DAC circuit11, and the output state of the resistance DAC circuit 13 in a samplingstate in the offset calibration operation. When the switch SW1 is openedand the switches SW2 and SW3 are closed, the standard voltage VCM isinput into not only the input of the capacitance DAC circuit 11 on theVCM side but also the input on the VIN side. The capacitors Cp16, Cp8,Cp4, Cp2, Cp1, and Cp_red are coupled to the VIN side of the capacitanceDAC circuit 11 through the switches Sp16, Sp8, Sp4, Sp2, Sp1, and Sp_redin the capacitance DAC circuit 11, and the standard voltage VCM isapplied. The VREFN output from the resistance DAC circuit 13 is appliedto the capacitor Cp_rdac. The electric charges sampled in this state areA/D-converted, and thus offset errors can be obtained. Namely, theoffset correction value e_dc[11:0] can be obtained.

As shown in FIG. 31, the sampling and the A/D conversion are repeated aplurality of times (for example, N times), so that N conversion resultse_dc_1 to e_dc_N can be obtained in the ADC_ECL[11:0], and the averagevalue of the remaining N−2 conversion results obtained by removing themaximum value and the minimum value is calculated to obtain the finaloffset correction value e_dc[11:0].

In the normal operation, the calculated offset correction valuee_dc[11:0] is added to the ADC_ECL[11:0] by the correction circuit 7shown in FIG. 28, and the result is output. Accordingly, the offset inthe characteristics of the ADOUT[11:0] is corrected.

[Sixth Embodiment] Mismatch Correction Calibration and OffsetCalibration

Both of the capacitance mismatch and the offset errors of thecapacitance DAC circuit 11 can be calibrated by combining the mismatchcorrection calibration according to the first to fourth embodiments andthe offset calibration according to the fifth embodiment.

FIG. 32 is a block diagram for showing a configuration example of an ADconvertor to which both of a capacitance mismatch calibration circuitand an offset calibration circuit are coupled in a signal processingcircuit 10 according to a sixth embodiment. The signal processingcircuit 10 includes the ADC 1, the ECL 2, a linearity calibrationcircuit 3_1, an offset calibration circuit 3_2, and the control circuit4, and further includes the switch SW1 on a route from the input voltageVIN to the ADC 1, the switch SW2 on a route from the standard voltageVCM to the ADC 1, and the switch SW3 between the input terminals of theinput voltage VIN and the standard voltage VCM of the ADC 1, as similarto the fifth embodiment. As shown in FIG. 32, the linearity calibrationcircuit 3_1 is arranged in the preceding stage of the offset calibrationcircuit 3_2, and the linearity calibration is performed first, so thatthe offset errors in the subsequent stage can be more accuratelycorrected.

An operation of the signal processing circuit 10 according to the sixthembodiment will be described.

FIG. 33 is a timing chart for showing an operation example ofcalibration of the signal processing circuit 10. When the calibrationoperation is started at Time t0, the switches SW1 and SW2 are closed andthe switch SW3 is opened first, and the correction value e11[11:0] ofthe Cp16 is calculated as similar to the description of the firstembodiment. Next, the switches SW1, SW2, and SW3 are controlled assimilar to the description of the fifth embodiment to calculate theoffset correction value e_dc[11:0]. In the normal operation after Timet3, the linearity is corrected using the calculated correction valuee11[11:0] of the Cp16, and the offset value of the offset correctionvalue e_dc[11:0] is corrected.

FIG. 34 is an explanatory diagram for showing an operation example ofthe calibration. In the case where a relation between the input voltageVIN of the signal processing circuit 10 and the output ADC_ECL[11:0] ofthe error correction logic 2 is as shown in the left graph, thelinearity is corrected using the correction value e11[11:0] of the Cp16first, so that a relation between the input voltage VIN and the outputADC_MISCAL[11:0] of the linearity calibration circuit 3_1 is correctedas shown in the middle graph. Each of the arrows shown in the middlegraph represents the correction value e11[11:0] of the Cp16. Next, theoffset errors are corrected using the offset correction valuee_dc[11:0], so that a relation between the input voltage VIN and theconversion output ADOUT[11:0] is corrected as shown in the right graph.Each of the arrows shown in the right graph represents the offsetcorrection value e_dc[11:0].

The linearity calibration circuit 3_1 may be any one of the calibrationcircuits shown in the first to fourth embodiments. For example, thelinearity calibration circuit 3_1 may be changed to a linearitycalibration circuit by which a plurality of bits is calibrated. Further,the correction values excluded from the averaging targets may be onemaximum value and one minimum value, or some large and small values.Further, a limiter may be added as described in the fourth embodiment.

[Seventh Embodiment] Pipeline-Type AD Convertor

In the first to sixth embodiments, an application example to thesequential comparison-type AD conversion circuit has been mainlydescribed. However, the present invention can be applied to an ADconversion circuit of a different system. In a seventh embodiment, anapplication example to an AD conversion circuit of a pipeline systemwill be described.

FIG. 35 is a block diagram for showing a configuration example of asignal processing circuit 10 in which a calibration circuit 3 is addedto an AD convertor (ADC) 1 of a pipeline system. The signal processingcircuit 10 includes the ADC 1, the error correction logic (ECL) 2, thecalibration circuit 3, the control circuit 4, and a standard voltagegeneration circuit 17.

The ADC 1 is an AD convertor of a pipeline system including L (L is aninteger equal to or larger than 2) stages (STAGE_1 to STAGE_L) 15_1 to15_L and an ADC 16 in the final stage. Aback-end ADC_i is formed bycombining stages (SATGE_i+1 to STAGE_L) 15_i+1 to 15_L subsequent to ani-th stage (STAGE_i) 15_i and the ADC 16 in the final stage. The ADC 16in the final stage is an ADC having a resolution of 2 to 3 bits.

The ECL 2 generates a conversion output ADOUT as the entire signalprocessing circuit 10 on the basis of an output from each stage of theADC 1 and an output ADC_RAW from the ADC in the final stage. Thestandard voltage generation circuit 17 generates a reference voltageVREF to be supplied to the ADC 1. The control circuit 4 controls apipeline operation of the ADC 1 and an operation of the calibrationcircuit 3. The calibration circuit 3 includes a multiplying circuit 19that multiplies a conversion output ADOUT by PN=±1, an averaging circuit5, and a correction signal operation circuit 18 that calculates acorrection coefficient of each stage on the basis of the correctionvalue output from the averaging circuit 5 to be supplied to each stageas a correction signal. In this case, PN is a constant that defines thepolarity in accordance with a state 1 and a state 2 to be describedlater. In the state 1, PN is +1. In the state 2, PN is −1 that means thereverse polarity. It should be noted that the averaging circuit 5 mayhave a circuit configuration similar to the averaging circuit describedin each of the first to fourth embodiments.

FIG. 36 is a block diagram for showing a configuration example of eachstage of the ADC 1. The i-th stage (STAGE_i) 15_i will be described. Thei-th stage (STAGE_i) 15_i is configured using a Sub_ADC_i21 thatA/D-converts an output of an i−1-th stage (STAGE_i−1) 15_i−1 of thepreceding stage, and an MDAC_i (20) that gives offset in accordance witha conversion result of the Sub_ADC_i (21) to an output of the i−1-thstage (STAGE_i−1) 15_i−1 of the preceding stage while multiplying Atimes and outputs the result. The amplification factor A is usually 2.The MDAC_i (20) includes an operation amplifier 23; a switched capacitoramplifier having a fixed capacitor Cf_i, a variable capacitor Cs_i, andswitches coupling these capacitors; and a D/A convertor DAC_i (22). Thevariable capacitor Cs_i can finely adjust the correction by thecalibration in accordance with a correction signal.

An operation of the signal processing circuit 10 will be described.

In the pipeline-type A/D convertor, a relative error between thecapacitance values of the fixed capacitor Cf_i and the variablecapacitor Cs_i (I=1˜L) in the MDAC_i (20) of each stage is largelyrelated to the accuracy of conversion as the A/D convertor. Accordingly,the signal processing circuit 10 performs calibration in which acorrection signal to finely adjust the variable capacitor Cs_i isdetermined so that Cf_i=Cs_i is satisfied in each stage.

In the calibration operation of the i-th stage (STAGE_i) 15_i, the VREFinstead of an input from the preceding stage is input from the standardvoltage generation circuit 17. In response to this, the sampling andconversion in the state 1 and the sampling and conversion in the state 2are alternately executed, and the A/D conversion at this time isperformed by the back-end ADC_i in the stage subsequent to the i-thstage (STAGE_i). FIG. 37 is an explanatory diagram for showing thestates of the switches at the time of the sampling and conversion ineach of the state 1 and the state 2 of the calibration operation. In thesampling, the fixed capacitor Cf_i and the variable capacitor Cs_i arecoupled to the ground level and the VREF in each of the state 1 and thestate 2, and electricity is charged to the VREF. In the conversion, thefixed capacitor Cf_i functions as a feedback capacitor of the operationamplifier 23, and the variable capacitor Cs_i functions as an inputcapacitor from the D/A convertor DAC_i (22) to the operation amplifier23 in the state 1. On the contrary, the variable capacitor Cs_ifunctions as a feedback capacitor, and the fixed capacitor Cf_ifunctions as an input capacitor in the state 2.

In the calibration operation of the i-th stage (STAGE_i) 15_i, thesampling and conversion in the state 1 and the sampling and conversionin the state 2 are alternately executed. The conversion output in thestate 1 and the conversion output in the state 2 are converted intodigital values by the back-end ADC_i, and are input into the calibrationcircuit 3 through the ECL 2.

The principle of the calibration will be described.

It is assumed that a relative error between the fixed capacitor Cf_i andthe variable capacitor Cs_i is α and Cs_i=(1+α)Cf_i is satisfied.

On the assumption that an output of the MDAC_i in the state 1 is Vout_i1and an output of the MDAC_i in the state 2 is Vout_i2,Vout_i1−Vout_i2=2α (VREF−VREFN) is satisfied. On the basis of the valuesof the Vout_i1, Vout_i2, VREF, and VREFN, a can be obtained, and thelevel of the correction signal to be supplied to the variable capacitorCs_i is determined on the basis of the values.

The Vout_i1 and Vout_i2 are output from the i-th stage (STAGE_i) 15_i inthe state 1 and the state 2, and thus are converted into digital valuesusing the back-end ADC_i in the subsequent stage to be input into thecalibration circuit 3 through the ECL 2. The value of the VREF suppliedfrom the standard voltage generation circuit 17 and the value of theVREFN output from the D/A convertor DAC_i (22) are already known becausethey are defined in the design stage, and the calibration circuit 3 cancalculate the value of a on the basis of these values. The state 1 andthe state 2 are alternately repeated a plurality of times (N times), sothat N pieces of α can be calculated. The N pieces of α are input to theaveraging circuit 5 as N correction values, and are averaged afterremoving the maximum value and the minimum value to be supplied to thecorrection signal operation circuit 18 as the final correction value.The effect of unexpected noise can be removed by removing the maximumvalue and the minimum value. The correction values excluded from theaveraging targets may be one maximum value and one minimum value, orsome large and small values. Further, a limiter may be added asdescribed in the fourth embodiment.

Accordingly, even in the calibration operation in which each stage ofthe pipeline-type AD convertor is corrected, when a large error isincluded in the correction value due to unexpected external noise andthe like, a more accurate correction value (correction signal) can becalculated by removing the effect.

The invention achieved by the inventors has been described above indetail on the basis of the embodiments. However, it is obvious that thepresent invention is not limited to the embodiments, but can bevariously changed without departing from the scope of the invention.

For example, the resolution of each of the ADC and the DAC, the positionof the redundant bit, the number of pipeline stages, and the likeexemplified in each embodiment can be appropriately changed or set.

What is claimed is:
 1. An analog to digital (AD) converter, comprising:an AD conversion circuit; and a calibration circuit that calibrates anoutput value of the AD conversion circuit, wherein the calibrationcircuit comprises: a right-shift circuit that shifts an accumulatedvalue of values obtained by removing a deviated value from a pluralityof output values of the AD conversion circuit, and wherein thecalibration circuit calibrates the output value of the AD conversioncircuit based on the shifted value.
 2. The AD converter according toclaim 1, further comprising an accumulation circuit that accumulates theplurality of output values of the AD conversion circuit, wherein theright-shift circuit shifts a value obtained by subtracting the deviatedvalue from the accumulated value.
 3. The AD converter according to claim1, further comprising an accumulation circuit that accumulates valuesobtained by removing the deviated value from the plurality of outputvalues of the AD conversion circuit, wherein the right-shift circuitshifts the accumulated value.
 4. The AD converter according to claim 1,wherein the deviated value includes at least one of a maximum value anda minimum value of the plurality of output values of the AD conversioncircuit.
 5. The AD converter according to claim 2, wherein the deviatedvalue includes at least one of a maximum value and a minimum value ofthe plurality of output values of the AD conversion circuit.
 6. The ADconverter according to claim 3, wherein the deviated value includes atleast one of a maximum value and a minimum value of the plurality ofoutput values of the AD conversion circuit.
 7. The AD converteraccording to claim 1, further comprising a limiter circuit that limitsthe shifted value, wherein the calibration circuit calibrates the outputvalue of the AD conversion circuit based on the limited value.
 8. The ADconverter according to claim 1, further comprising: a limiter circuitthat limits the plurality of output values of the AD conversion circuit;and an accumulation circuit that accumulates values obtained by removingthe deviated value from the limited values, wherein the right-shiftcircuit shifts the accumulated value.
 9. The AD converter according toclaim 7, wherein the deviated value includes at least one of a maximumvalue and a minimum value of the plurality of output values of the ADconversion circuit.
 10. The AD converter according to claim 8, whereinthe deviated value includes at least one of a maximum value and aminimum value of the plurality of output values of the AD conversioncircuit.